Figure 5. represents the under side of the all or nothing, with the two studs ; one u as a centre on which it moves, and the other x on which the star and snail turn, Fig. 9.: the hole c of this piece allows the square of the fusee of the movement to come through, and. lastly, passing through the dial, serves for winding up the watch. W, Fig. 3. is the locking spring and bolt ; this is what prevents the movement from opening out of the case.
Y is a small cock or bridge which keeps the rack to its place, and prevents it from getting away from the plate, permitting it to turn only on its own centre.
All the parts of the repetition or motion work, w hich have been described, are placed on the back of the pillar plate, and are covered by the dial ; so that between the plate (Fig. 3.) and the dial there must be an interval, to allow sufficient play for the motion-work : It is for this purpose that a piece is destined, which is not represented here, and which is called the brass edge. This is a sort of circle, or ring, into which the circumference, or edge of the pillar plate, is sunk a little way, with which it is kept fast by means of keys, or grills 13. and 14. The brass edge is co vered by the dial, fixed after that of the brass edge, by means of a screw.
A repeater is made to strike the hour which it indicates the moment we press in the pendant ; so that the machine must be contrived in such a way, that it may be easy to push in the pendant, and that the blows of the hammers may be the strongest possible. With respect to the first,
that depends on two things ; the given force of the spring, and the length of the pusher ; that is to say, the space de scribed, and the manner of making the pusher act on the rack. With regard to the last, the rack must be placed in such a manner, that the point of contact of the pusher fol lows the arc described by the rack, in such a way that the force shall not be decomposed, so that the action of the hand on the pusher shall act wholly upon the rack.
With regard to the pusher, its length depends on the point where it acts on the rack ; that is to say, according as it acts farther or nearer the centre of motion. It is obvi ous, that if it acts near the centre, more force is required, and it will describe a less space, and vice versa. As to the force of the blow of the hammer, it is limited by the force of the repeating main-spring, and by the force that the run ners require to move or keep them in motion ; for it is clear, that it is only the excess of the force of the spring over the resistance of these wheels that can be employ ed to raise the hammer. The number of blows of the hammer, for one revolution of the ratchet, determines again the force of the blow.
On Compensation Pendulums.